1. Field of the Invention
The present invention relates to filling devices and methods for the filling of shells of prosthetic devices, such as mammary and penile implants.
2. Background Information
The preferred embodiment of the present invention is intended for the filling of mammary prostheses with a new, alternate polymeric filling material. While other types of polymers have been successfully filled into an implantable device, this alternate gel, because of its own special characteristics, called for the reevaluation of conventional filling equipment and techniques, and the development of new and unique equipment and techniques to assure the desired end result. The polymer in the alternate filler is a cross linked (poly) acrylamide, highly viscous (.about.130-240K centipoise), cohesive (the ability to maintain structure within its domains upon placement of stress), and shear sensitive (non-Newtonian). The polymers physical characteristics related to viscosity, molecular weight distribution and number and flow characteristics are at their peak during fill operations.
In the design and implementation of a novel mammary prosthesis filling device/system, several critical attributes must be resolved. These attributes include: (1) the filling of implantable devices with the alternate gel in a "bubble free" physical state which allows optimum conditions for radiolucency analysis; (2) a process which provides consistently accurate weight or volume dispensing of a gelatinous material; (3) a device which simultaneously pulls a vacuum on an implant shell to evacuate all contained air therein; and (4) minimizing the shearing effect associated with kinetic pressure typical of current filling systems.
Expanding on these attributes, it is important to recognize that the ability to fill a device "bubble free" is critical, as it relates to the ability of a surgeon or appropriate hospital staff member to accurately interpret x-rays for symptoms associated with breast cancer, tumor growth, device wrinkles, capsular contraction(s) and the like. A bubble in the device will show as a spot on an x-ray, leading to potential misdiagnosis or unnecessary medical concern.
In filling of the device, a specification of .+-.2.0% (by weight or volume) is critical, as a mammary prosthesis filled with the alternate gel cannot be weight or volume adjusted once it has been implanted (unlike saline filled prostheses). The design and size of the pump is critical in meeting the requirements of the specific polymer being used. Any positive-displacement pump (piston, rotary lobe, rotary piston, progressive cavity, peristaltic, etc.) will function properly with low viscosity Newtonian fluids. With shear sensitive fluids, the pump design is restricted mostly to peristaltic, progressive cavity, rotary lobe or rotary piston pumps. The calibration of the pump flow rate (speed of cycles) piping or tubing (type, ID, OD), pressure (polymeric force) and filling speed are all critical in meeting the stringent specification, as practically and efficiently as possible.
Providing an "air-free" environment in the prosthetic device shells to be filled serves two purposes: the first being that the alternate filler will never contact outside air sources which are known to carry contaminants, and the second being that upon filling with the gel, pocketed void areas that could encapsulate air are not present, thus preventing areas in the device which would form air pockets and have to be evacuated by alternate methods.
The final area, and one of the most critical, is the shear thinning of the polymer as it is being injected into the mammary device (shell). The polymer of the alternate gel is non-Newtonian (viscosity breakdown with increased shear rates) and must be handled in a system that prevents excessive shearing and introduction of unnecessary heat. While the polymer will eventually be subjected to the forces associated with steam sterilization (heat and pressure), excessive physical deformation is not acceptable in maintaining a final polymeric viscosity to provide the desired "feel".
Filling of devices comparable to the mammary and/or penile devices currently manufactured by Mentor Inc., assignee of the present invention, range from syringe injection to filling ports. With polymers such as silicones, the viscosity of the polymer is 50-75% lower at the filling operation than the alternate filler considered herein.